Conventional perforating gun sections used in perforating well casings typically include charge carriers designed to support several separate perforating charges within the desired longitudinal spacing and sometimes a desired radial orientation. Examples of various convention perforating gun sections are illustrated in U.S. Pat. No. 5,095,999 issued to Daniel C. Markel on Mar. 17, 1992, the specification of which is incorporated herein in its entirety. In particular, the Markel patent illustrates a conventional enclosed perforating gun section having a plurality of perforating charges mounted on a carrier strip and enclosed and protected within a carrier tube. (See U.S. Pat. No. 5,095,999, Column 5, lines 20-39 and FIG. 5.)
As is well known in the industry, perforating gun sections use perforating shaped explosive charges designed to shape and direct the explosion with great precision along the focal axis. Typically, a perforating shaped charge will shape and direct a liner material to create a uniform circular jet that is highly focused and directed along the focal axis. The focused jet penetrates the casing that lines the well bore and the surrounding geological formation. The detonation of the perforating charges is intended to increase production of the well, which is hoped will result in a substantial increase in production pressure at the well head.
Usually, maximizing the perforations achievable in a single-shot downhole procedure is desirable. For example, it is sometimes desirable to perforate hundreds even thousands, of linear feet of downhole casing to enhance well production. However, the length of the typical perforating gun section is about thirty feet. Of course, it is possible to achieve increased perforation of the downhole well casing by repeating the procedure of lowering a perforating gun section to perforate the downhole well casing and retrieving the spent perforating gun section until the desired longitudinal portion of the downhole well casing has been perforated. However, the time and expense involved in repeating each such downhole procedure mitigate in favor of perforating the desired portion of the well bore in a single downhole procedure. Thus, if it is desirable to perforate such lengths of the downhole casing, as is frequently desirable, two or more perforating gun sections must be connected together. The assembled string of perforating gun sections is then lowered downhole to perforate the well in a single shot.
In the past, conventional threaded pin-and-bell connectors have been used to connect perforating gun sections. For example, after a first perforating gun section is positioned and set in a slip assembly at the rig floor of a well (usually with a threaded pin connector at the upper end thereof), a second perforating gun section is picked up and brought into position over the first perforating gun section. As the second perforating gun section (usually with a threaded bell connector at the lower end thereof) is swinging in the blocks of the rig, it must be carefully axially aligned with the first perforating gun section so that it can be set on the pin connector of the first perforating gun section. The second perforating gun section is then rotated to make up the threaded connection.
There are several problems of using threaded pin-and-bell connections. For example, the process of carefully aligning and threading one elongated perforating gun section to the next is time consuming. Skilled oil-field hands need about one to two minutes to make up or break apart perforating gun sections using threaded pin-and-bell connectors. The step of aligning the second perforating gun section can be particularly difficult in windy conditions, which cause the thirty-foot section to swing in the blocks. If the second perforating gun is not properly aligned, the threads of the pin-and bell connectors are likely to gall and bind.
Furthermore, connecting perforating gun sections with such conventional threaded pin-and-bell connectors presents special problems and risks. For example, manually rotating the second perforating gun section with a hand wrench is more time consuming than the with the use of power tongs. With a hand wrench, however, the oil-field hands can feel the process of threading the connector and be more sensitive to whether the threads are properly aligned to prevent galling. But while the use of power tongs to rotate a perforating gun section to make up the threaded connection is faster, if it works, the threads of the connection are much more likely to gall because of the speed of rotation and the oil-field hands' inability to feel the threading and make any necessary adjustments in the alignment of the threads.
A galled threaded connector for perforating gun sections presents particular problems and dangers because of the explosives used in the sections. For example, if the threads gall and bind in a threaded pin-and-bell connector between two perforating gun sections, the transmission of the detonating signal between the two sections may not be reliable. Thus, it is usually desirable or necessary to separate the galled connection, and replace the connector and possibly both the perforating gun sections. However, unthreading the galled threads of the connector is sometimes difficult or impossible. Furthermore, cutting or shearing galled perforating gun sections, which contain high explosives, is counter indicated for obvious safety concerns. Thus, a galled threaded connection between perforating gun sections presents a serious problem. In the past, one of the only solutions to the problem of a seriously galled threaded connection has been to raise the two galled perforating gun sections and unthread the lower connection from the remainder of the perforating gun string, to then safely remove and handle the two improperly joined sections. However, this is wasteful of expensive perforating gun section equipment and extremely time consuming.
For these reasons, it can take several minutes to align, set, and manually make-up each threaded connection between the perforating gun sections, and a galled connection can seriously impede the process of perforating a well. Thus, there has been a long-felt need for a better, more reliable, and faster connector for perforating gun sections.
As an alternative to conventional threaded pin-and-bell connectors, some perforating gun connectors are activated or released by certain types of rotational movements other than threading. However, it is becoming increasingly common to use perforating gun sections with coil tubing. Coil tubing may be hundreds or thousands of feet long, such that it is extremely difficult or completely impractical to attempt to rotate the coil tubing to operate a latch or release connection. Thus, it would be desirable to provide a latch and release connector for use with perforating gun sections that does not have to be rotated.
Additional problems are encountered in using perforating guns through a blowout preventer. The typical drilling well is provided with a blowout preventer ("BOP") at the well head, which is intended to maintain any pressure within the well head and prevent a blowout of the well. A blowout preventer is also used for safety to recomplete an existing well. A blowout can be an extremely hazardous situation if the oil or gas explodes or catches fire. Furthermore, even if the oil or gas does not ignite, allowing such uncontrolled escape is extremely wasteful of a valuable resource and harmful to the environment. In some countries such as the United States, an uncontrolled escape can subject the producer to substantial government fines for the environmental pollution and the costs of environmental clean up. Blowout preventers are well known in the art, and represented, for example, by U.S. Pat. No. 4,416,441 entitled "Blowout Preventer" issued to Denzal W. Van Winkle on Nov. 22, 1983 and by U.S. Pat. No. 4,943,031 entitled "Blowout Preventer" issued to Denzal W. Van Winkle on Jul. 24, 1990, both of which patents are incorporated herein by reference in their entirety.
According to the art, two or more blowout preventers are typically used in a stack at the well head. For example, the rams of a lower blowout preventer are employed as slip rams, which have serrated metal teeth for gripping and holding a section of downhole tubing or other tool. The slip rams are useful as a type of slip assembly for holding a section of downhole tubing or perforating gun section, which can have many additional sections connected to and suspended from the lower end thereof. The rams of a second blowout preventer above the first are employed as sealing rams, having rubber seals adapted to be compressed against the downhole tubing or other tool to form a pressure-tight seal around the tubing or tool.
Having additional blowout preventers in the stack is common. For example, the rams of a third blowout preventer above the sealing BOP can be equipped with shearing blades for cutting a piece of tubing for which the threads have seized onto the next tubing and cannot be normally unthreaded. The rams of a fourth blowout preventer above the rest can be employed as a blind seal, such that the well head can be completely sealed. Thus, a production well usually has at least two blowout preventers at the well head used for controlling the well.
However, working through a stack of blowout preventers presents several additional problems and challenges. This is true even though the pressure at the well head is initially substantially balanced such that the well head can be opened for the insertion of a perforating gun section. For example, after using the perforating gun section to perforate the downhole well bore, it hopefully increases the well production and the production pressure at the well head. Thus, a problem is then presented of how to withdraw the spent perforating gun section through the blowout preventer. The problem is particularly problematic because a spent perforating gun section has itself been thoroughly perforated by the detonation of the perforating shaped charges. For example, the sealing rams of the sealing blowout preventer may have difficulty fully sealing against the warped, twisted, and punctured metal of the perforating gun section. Furthermore, the open holes created in the spent perforating gun section provide multiple conduits for the pressurized fluid in the well beneath the blowout preventers to enter the spent perforating gun section. Thus, the spent perforating gun section provides an undesired conduit through the blowout preventer stack, leaking or spewing the pressurized production.
A prior art method of addressing this problem of how to remove a spent perforating gun section has been to balance the pressure in the well. Balancing the pressure is normally accomplished by pumping the appropriate density of drilling mud into the well head to equalize the pressure below and above the well head. However, this balancing procedure is sometimes called "killing" the well because it inhibits the production and can create other pressure management and technical difficulties. There has been a long-felt need for an apparatus and method for withdrawing the spent perforating gun section through the stack of blowout preventers at the well head without having to even temporarily kill the enhanced well production.
Furthermore, enhancing the well production of a well that has some positive well pressure at the well head is often desirable. In such a case, perforating the downhole casing is still desirable. Of course, working through a blowout preventer stack with an intact perforating gun section before it has been detonated can be accomplished by employing a lubricator above the blowout preventer stack. The perforating gun sections can be made up with the lubricator according to techniques well known to those of skill in the art. However, the use of a lubricator above the blowout preventer further limits the length of the perforating gun sections that can be used to the practical length of the lubricator. A typical lubricator for such applications can accommodate perforating gun sections of up to about 35 feet (11 meters).
Unfortunately, the use of conventional threaded pin-and-bell connectors through a lubricator above a blowout preventer stack is particularly time consuming. For example, it typically requires about five minutes for skilled oil-field hands to make up perforating gun sections together through a lubricator above a blowout preventer stack. There has been a particular long-felt need for an apparatus and method that would permit much faster connection and release of perforating gun sections through a lubricator and blowout preventer stack. The cost of oil field hands and recovered production time involved in stringing several perforating gun sections together has driven the search for faster apparatuses and methods. Nevertheless, to the knowledge of the inventors there is still a great need for additional improvements and methods.
In some applications, perforating gun sections and connector assemblies must be able to pass through reduced diameter tubing or other downhole restrictions to reach the location in the casing where the perforation is to be performed. In these applications, the axial cross-section profile of the perforating gun string is particularly important. For example, in the perforation of a five-inch casing, passing through a small bore may be necessary for the perforating gun assemblies, such as two-and-one-half inch or one-and-eleven-sixteenth inch tubing or other passageway. These through-tubing perforating gun assemblies can be characterized as low-profile assemblies because of the restricted passageways through which they must pass to reach the desired downhole perforation location. These low-profile perforating gun assemblies do not have the luxury of design spacing which is present in perforating gun assemblies whose maximum outside dimensions approximate that of the casing that is to be perforated. These small profile or through-tubing perforating gun assemblies present particular problems that are not present in their larger profiled cousins.
Thus, there has been a long-felt need for assemblies and methods capable of more quickly stringing two perforating guns together for firing in a single downhole procedure, thereby reducing the time and expense involved in perforating a well. There has been a long-felt need for apparatuses and methods of withdrawing and more quickly separating spent perforating gun sections from a well. In addition, there has been a particular need for apparatuses and methods for connecting and separating perforating gun sections through a blowout preventer stack while maintaining the pressure below the blowout preventer stack.